How to make sure the final bone in an IK chain doesn't rotate?
There are two ways that you can do this.  You can either select the bone and add in rotation constraints or you can do it a really easy way.  Simply select the bone you want not to rotate while in pose mode.  Then press N to bring up the additional menu on the right side of the view port.  The top tab that says transform is what you want to look at.  Where is says rotation, select all of the locks and this will prevent that bone from ever rotating.    Hope this helped!